Looming Hurricane Earl forced vacationers and some residents off North Carolina's barrier islands on Wednesday, churning up dangerous swells and prompting storm alerts along the U.S. east coast.

Watches and warnings were posted along the Atlantic coast for most of North Carolina, Virginia, Maryland and part of Delaware, alerting residents that hurricane conditions were possible in 36 to 48 hours.

Earl was a major Category 3 hurricane with 125 mph (205 kph) winds as it barrelled across the Atlantic east of the Bahamas, the U.S. National Hurricane Centre said.

It showed signs of strengthening and could become a Category 4 storm on the five-step Saffir-Simpson scale of intensity, the centre said.

While a direct U.S. landfall was not forecast, Earl was on track to deliver a sidelong blow to the North Carolina coast, with its centre passing within 100 miles (160 km) of the state's Outer Banks barrier islands late on Thursday or early on Friday.

We're forecasting it still to be a major hurricane at that time so this is still a serious situation, hurricane centre forecaster Wallace Hogsett said.

Earl was expected to bring driving rain, high winds, pounding surf and rip currents to the densely populated U.S. coast from North Carolina to New England during the Labour Day holiday weekend marking the end of the summer vacation season.

No storm had threatened such a broad swath of the U.S. shoreline since Hurricane Bob in 1991, hurricane centre spokesman Dennis Feltgen said.

Based on the storm's projected path, the U.S. Census Bureau estimated 1.8 million people in North Carolina and Virginia could feel Earl's effects in the next two days.

President Barack Obama was briefed on storm preparations, Federal Emergency Management Agency Administrator Craig Fugate said.

EVACUATIONS ORDERED

Earl was a large storm with hurricane-force winds extending 90 miles (150 km) from the centre and tropical storm-force winds extending out 200 miles (325 km).

Forecasters said a low pressure trough moving out of the southwest over the United States would push the hurricane to the northeast on Thursday, keeping it off the U.S. Coast.

If the turns occur a little bit later than we're forecasting it could bring more of a threat at that time to extreme eastern Long Island or southeastern New England, Hurricane Centre Director Bill Read told reporters in the conference call.

U.S. and Canadian East Coast oil refiners said they were monitoring Earl but that it was too early to begin to taking precautionary measures.

Earl posed no threat to U.S. oil and gas installations in the Gulf of Mexico.

At 2 p.m. EDT (7 p.m. British time), Earl was moving across the Atlantic well to the east of the Bahamas and was about 680 miles (1,095 km) south-southeast of Cape Hatteras, North Carolina, the National Hurricane Centre in Miami said.

North Carolina's Dare County ordered the mandatory evacuation of all visitors in Hatteras Island, a picturesque vacation spot that draws large numbers of tourists each year. Officials said high waves could wash over the coastal highway, stranding residents.

Vacationers and residents jammed onto ferries bound for the mainland after evacuations were ordered on Ocracoke Island, another of North Carolina's Outer Banks that jut into the Atlantic Ocean.

On Hatteras Island, visitor John Gusciora of Leesburg, Virginia, was reluctant to leave the village of Salvo, where he had rented a cottage with three families. The weather was sunny on Wednesday, with little to no breeze.

It's disappointing. We've been coming down here for years and this is the best weather we've had until today. Now we have to try to pack up our bags and beat the rush off the island, Gusciora said.

It's sad to go, but it's better to be safe, he added.

It was too early to predict how close the hurricane would come to New York when it churned offshore east of the city over the weekend, but emergency officials were recommending that local authorities and residents remain alert.

INSURED LOSSES IN CARIBBEAN

Earl caused $50 million (£32.4 million) to $150 million in insured losses in the Virgin Islands, St. Maarten, St. Martin and Puerto Rico when it blew through the Leeward Islands of the Caribbean earlier this week, risk modelling company AIR Worldwide said.

Behind Earl, Tropical Storm Fiona was about 100 miles (160 km) east-northeast of the northern Leeward Islands and strengthened on Wednesday, with winds up to 60 mph (96 kph) as it moved northwest, the forecasters said.

Fiona was expected to curve north to northeast in the Atlantic and keep far away from the U.S. coast while remaining a tropical storm at its peak intensity.

Meanwhile, a tropical depression formed in the Atlantic 830 miles (1,335 km) west-southwest of the Cape Verde Islands and was expected to become Tropical Storm Gaston in the next day or two. It was forecast to move west towards the northeast Caribbean islands.

(Writing by Jane Sutton; Additional reporting by Tom Brown and Pascal Fletcher in Miami, and Eileen Moustakis in New York; Editing by Pascal Fletcher and Cynthia Osterman)

Hurricane